The present invention relates to testing a stereo audio television system, and more particularly to a two-tone test signal method for calibrating a monitoring system for a BTSC broadcast system which compensates for high frequency noise when the low frequency range of the monitoring system is tested.
The BTSC stereo audio system for broadcast television combines left and right audio channels into L+R and L-R components. The L+R component is transmitted in a conventional manner as a baseband Main (M) sum component, and the L-R component is transmitted as a double sideband, suppressed carrier signal at a frequency above the L+R component as a Difference (D) component as shown in FIG. 1. The L+R component is processed in a linear manner for both transmission and reception, but the L-R component is "companded" in a nonlinear manner for noise reduction, i.e., compressed before transmission and expanded after reception. To recover the original left and right audio channels, the sum and difference components are added and subtracted. Proper recovery is critically dependent upon recovering the sum and difference components with exactly correct phase and amplitude.
For stereo audio poor quality is demonstrated most obviously by lack of channel separation. Separation is defined as the amplitude ratio between channels when only one channel is excited. This signal channel separation is expressed in terms of decibels, such as -60 dB for a typical value. To measure the performance of the compression and transmission system, a monitoring system requires an expander which accurately implements the BTSC specification to accurately recover the L-R component from the transmitted D component.
One obvious method of calibrating the monitoring system is to introduce a single test tone at each of several representative audio frequencies and to determine the response of the system at each frequency. Due to the nonlinear nature of the compression and expansion, the low and high frequency portions of the D component are subjected to different amounts of compression and expansion as illustrated in FIG. 2. For ideal theoretical situations such a single tone test calibration system would be sufficient, but in the real world there is a certain amount of noise present across the frequency spectrum introduced by the electronic components themselves. For high frequency test tones the noise has virtually no impact. However for low frequency test tones the high frequency component of the noise has a significant effect. Even nose at 80 dB below the test tone causes the "spectral" expander to alter the phase and amplitude of the processed signal so that stereo separation is reduced and the measurement of the stereo separation of an actual signal is in error. The effect of noise is illustrated graphically in FIG. 3 where the effect of noise at 10 kHz on the phase, expander gain and separation limit for a 300 Hz test tone is shown. Since the noise is unpredictable, the net effect is to decrease the ability of the monitoring system to accurately measure signal separation. It is very difficult to realize a signal-to-noise ratio (S/N) high enough to avoid this effect.
It should be noted that the high frequency noise problem does not arise in an actual BTSC system receiving a broadcast signal since the noise at the output of the BTSC compressor at the transmitter in the absence of high frequency input signals is large compared with that added by the receiver and expander circuits. Thus the compressor and expander are "seeing" the same amount of noise and there is no mistracking. However when the expander is seeing noise that the compressor does not see, such as simulated test signals from a "perfect" noise free compressor, then any added noise ahead of the expander causes the mistracking as described above.
Therefore what is desired is a method for testing a BTSC stereo audio monitoring system which keeps unpredictable and uncontrolled noise from affecting the performance measurements.